Computers often generate video images, such as for games or graphical displays, by storing a single image in memory which represents a background scene in which various foreground objects appear. For example, the background may be outer space with stars and planets and the foreground may be a series of objects such as space ships and meteors which move across space. More sophisticated computer displays use two video signals separately representing the background scene and the foreground objects. The background video may be computer generated, e.g. derived from images stored in one part of the computer's memory, produced by playback of scenes stored on a video disk, or generated from a data source. The foreground objects or "sprites" typically are independently generated by the computer and stored in another part of the memory.
The overlay of the foreground video objects is accomplished by switching between the two video sources so as to replace portions of the background image with the foreground objects. Techniques similar to this have been used in television broadcasting to key or insert titles into a picture. However, this process tends to produce sharp edge transitions between the foreground object and the background scene, which often results in an unnatural appearing composite image, i.e. the foreground objects stand out from the background rather than blending into it.
This unnatural appearance is aggravated further by the finite size of the picture elements (pixels) used to form the objects in the computer generated foreground image. The finite pixel size often results in curved or diagonal edges of objects appearing jagged, an effect commonly referred to as aliasing. These jagged edges also contribute to the appearance of the foreground object standing out from the background and thus rendering the final composite image less natural in appearance.